1. Field of the Invention
This invention relates to the treatment of carbonaceous materials with hydrogen to form hydrocarbon liquids and gases suitable for conversion to fuels.
More particularly, this invention relates to reacting solid pulverized carbonaceous materials, such as coal, with heated hydrogen to from hydrocarbon liquids and gases suitable for conversion to fuels or for use as a chemical feedstock.
2. Description of the Prior Art
It is generally well known the conversion of coal to liquid or gaseous fuels is achieved by the addition of hydrogen. This may be accomplished by the direct contact of coal with hydrogen as in the Bureau of Mines Hydrane process to produce methane; by a catalyzed liquid-phase reaction with hydrogen to produce liquid products as in the Synthoil process; or indirectly by reacting coal with steam. Many different processes have been proposed and are under development. These schemes vary in the method of contacting coal with hydrogen or steam, and in the type of coal feed utilized. A solid, such as coal, can be contacted with a gas in three basically different ways. In the first, gas is forced through a fixed or slowly moving bed of solid. Another method of contact is by use of a fluidized bed. With sufficiently small solid particles and a sufficiently high gas velocity in vertical upward flow, the aerodynamic drag forces on the individual particles begin to approach the gravitational forces and the particles themselves begin to move about. The bulk properties of the gas solid mixture then become those of a fluid. Because of the improved heat and mass transfer characteristics in a fluidized bed as opposed to a fixed bed, most coal gasification processes now are the fluidized bed variety. Yet another basic category of gas solid contacting is entrained flow as in the Bigas process. In this regime, gas velocities are high enough and particle sizes small enough that the solid particles are carried along with the gas stream. An advantage of the entrained flow processes is the ability to utilize any grade or class of coal. Caking coals will agglomerate causing difficult problems when fed to fluidized or fixed bed systems. Further advantages of entrained flow with respect to gas production include operation at high temperatures so that tar production is kept to a minimum, adaptability to slagging conditions and high energy production per unit volume. The present invention is applicable to all of these types of processes, but is particularly applicable to an entrained flow coal conversion process.
U.S. Pat. No. 3,030,297 describes a process which comprises heating dry particles of coal entrained in a heated stream of hydrogen at total pressure of about 500-6000 psig from a temperature below about 300.degree. C. to a reaction temperature in the range of from about 600.degree. C. to about 1000.degree. C. Two minutes are required to heat the coal particles to about 600.degree. C. and then two to twenty seconds time at temperature for hydrogenation. The slow heat-up results from the main hydrogen stream being utilized to carry the coal into the reactor. The products of reaction are then cooled below reaction temperature to provide a product comprised of light oil, predominantly aromatic in nature, and hydrocarbon gases, primarily methane, ethane, and carbon monoxide.
A disadvantage of this process is that the coal particles entrained in the hydrogen are preheated prior to introduction into a heat chamber; thus, the reaction process is started upstream of the reaction chamber which may cause agglomeration and plugging within the conduit carrying the entrained coal. The present invention overcomes this agglomeration problem by providing two sources of gas. One source of gas, such as hydrogen, brings entrained coal into an injector at ambient temperature, and a separate source provides heated hydrogen to an injector which contacts the entrained dense phase coal downstream of an injector within a reaction zone, thereby starting the hydrogenation process within the reaction chamber and not upstream of the chamber.
A further disadvantage of the process shown in U.S. Pat. No. 3,030,297 is that it calls for the transfer of heat to the entrained coal particles through a tube wall. At the mass throughputs specified in the example, it is doubtful that enough heat could be transferred through the tube wall in a reasonable length to sufficiently heat the coal and, at the same time, use the tube wall to contain the system pressure. This type of reactor does not scale to the necessary larger diameters for commercial coal conversion reasonably because the heat transfer surface-to-volume ratio decreases rapidly with an increase in size.
Another patent, issued to Schroeder et al (U.S. Pat. No. 3,152,063), teaches a process which comprises dispersing pulverized and catalyzed coal, in the absence of a pasting oil, in hydrogen under a pressure of about 500 to 4000 psig, reacting the mixture of coal and hydrogen at a temperature in the range of about 450.degree. to 600.degree. C., for a gas residence time of less than about 200 seconds, cooling the reaction products and recovering liquid and gas hydrocarbon products therefrom.
Schroeder teaches passing of catalyzed coal and hydrogen into a two-stage reactor that consists of a multiplicity of parallel tubes axially extending within the reactor. The tubes are heated by a source of hot gas to start the reaction within the tubes. Vaporized oil and gas products are drawn off as well as unused hydrogen to a cooling device. The residual heavier oil and tar products are collected in the bottom of the reactor and a source of hydrogen may then be brought in to further hydrogenate these heavier products.
A disadvantage of this invention is that the pulverized coal must be passed through a catalyzing process, sent through a dryer and grinder and finally separated into minute particles by passing the coal through a screening process. The present invention utilizes finely-divided pulverized coal directly without the foregoing pre-treatment process. A further disadvantage of the prior art process is that it also utilizes the carrier hydrogen in the coal passages as the main source of hydrogen. The heat-up process then takes considerable time as compared to the present invention in that the carrier gas cannot be preheated prior to entering into a reaction chamber.
U.S. Pat. No. 3,960,700 suggests a process for treating carbonaceous material with hydrogen in the absence of an added catalyst. In accordance with the process disclosed therein a liquid or crushed solid carbonaceous material is added to a reactor where it is contacted with hot hydrogen in an amount to provide a hydrogen-to-material ratio varying from about 0.05 to about 4.0. The hydrogen and the carbonaceous material are reacted at a temperature from about 400.degree. C. to about 2000.degree. C. and a pressure of from about 3.4 to about 34 megapascals (500 to about 5000 psig). The reaction temperature is maintained by heating the hydrogen introduced to a temperature of about 50.degree. C. above the desired reaction temperature. The reaction products are rapidly quenched to provide a total residence time of the reactants within the reactor of from about 2 milliseconds to about 2 seconds. This patent contains no specific teaching with regard to how the hydrogen is heated, referring only to "well known" processes. Presumably, therefore, the patent suggests conventional means such as indirect heat exchangers, electrical resistance heaters and the like.
U.S. Pat. No. 3,963,598 suggests a process for the flash hydrogenation of coal. In accordance with the process disclosed therein, substantially dry powdered coal having a particle size in the range of from about 50 to 500 microns is contacted with hydrogen gas at a temperature to produce a reaction temperature between about 500.degree. C. and 800.degree. C., and a pressure in the range of from about 6.9 to 28.4 megapascals (68 to 280 atmospheres). The reactants are contacted in a rotating fluidized bed for a coal residence time of not in excess of 5 seconds and hydrogen contact time not in excess of 0.2 seconds to produce liquid hydrocarbons which are rapidly cooled to a temperature sufficiently low to prevent further cracking of the liquid products. The only teaching of the method for heating hydrogen is a general reference to a hydrogen heating furnace and a statement in the example is the hydrogen temperature should not be over a 1000.degree. C. based on material limitations. Thus, this patent also contemplates conventional heating techniques.
In U.S. Pat. No. 3,997,423 there is discussed another process for a short residence time, low pressure hydropyrolysis of carbonaceous material. In accordance with the process disclosed therein, crushed coal is mixed with hot hydrogen at 500.degree. C. to 1500.degree. C. and 0 to 1.7 megapascals (250 psig) in a reactor and then, after a short reaction time, the reaction products are rapidly quenched. The total heat-up, reaction, and quench time is less than 2 seconds. It is alleged that this short residence time results in a high yield of coal tars. It is stated that "the heart of the invention resides in the concept of a short total residence time of the carbonaceous material in the reactor, at a low pressure between about atmospheric pressure and 250 psia." While this patent suggests the use of high temperature hydrogen as a means for maintaining and controlling the reaction temperature, it suggests no specific means for heating the hydrogen, and further teaches that the inlet hydrogen temperature should be approximately 50.degree. C. higher than the desired reaction temperature.
More recently, in U.S. Patent Application Ser. No. 871,163 filed Jan. 20, 1978 and assigned to the Assignee of the present invention, there is suggested another coal liquefaction method and apparatus to produce hydrocarbon liquids and gases in accordance with the method disclosed therein. Pulverized coal particles entrained in a gas in a dense phase are injected into a reaction chamber at ambient temperature, a separate source of hydrogen at elevated temperature also is injected into the reaction chamber to raise the temperature of the coal, a portion of the hydrogen reacting with the coal to provide hydrogenation products which are rapidly quenched and collected. The total reaction time generally is in the range of from about 10 to 500 milliseconds.
Although the chemistry of coal pyrolysis and hydrogenation has been apparent for some time, no commercial scale reactor exists which efficiently utilizes the rapid-reaction regime. Some of the basic reasons for this appear to be a lack of adequate gas/solid injection and mixing technology, difficulty in meeting chemistry and residence time requirements, and agglomeration and plugging of the reactor. Hydrogenation of raw bituminous coal usually results in agglomeration, so that typical fluidized bed or moving bed reactors cannot be used as heretofore described. In addition, the requirement of short residence time (less than 1 sec) necessarily restricts the reactor to an entrained flow type. By maintaining rapid mixing, heat-up, and reaction of the coal near the point of injection and by maintaining hot reactor walls, the agglomeration problem can be avoided as taught in the aforementioned pending application.
Another significant disadvantage of coal hydrogenation processes of theentrained flow type is the amount of gas which must be heated to provide and maintain the desired temperatures in the reaction zone. More particularly, the temperature of the inlet gas must be maintained below about 1100.degree. C. and generally below about 1000.degree. C. to avoid the necessity of using exotic and expensive high temperature alloys for the materials of construction. Thus, a substantial amount of gas must be heated to maintain, for example, a temperature of around 650.degree. to 950.degree. C. in the reaction zone. Since only a small amount or portion of hydrogen introduced actually reacts with the coal, the economics of the process further require that the excess hydrogen be collected for recycling. In addition, the power requirements for transferring, collecting and compressing gas streams are substantial. Indeed, one of the principal obstacles standing in the way of a commercial process for the conversion of coal to valuable hydrocarbon products is that the energy required for conversion frequently amounts to from about 30% to 50% of the energy available from the coal. Clearly, therefore, there is a need for a process which is capable of converting coal into valuable liquid and gaseous products in which the energy requirements for such conversion are less than about 30% of the energy of the coal, and preferably less than 25%.